How to Use CM108B: Examples, Pinouts, and Specs

The CM108B, manufactured by C-Media, is a USB audio controller designed to deliver high-quality audio input and output capabilities. It integrates a USB interface with audio codec functionality, making it an ideal solution for computer audio applications. The CM108B supports various audio formats and features a built-in DAC (Digital-to-Analog Converter) and ADC (Analog-to-Digital Converter), ensuring excellent audio performance. Its compact design and versatile functionality make it suitable for multimedia devices, USB headsets, sound cards, and other audio peripherals.

• USB sound cards
• USB headsets and microphones
• Multimedia devices
• Audio recording and playback systems
• Embedded audio solutions

The CM108B comes in a 48-pin LQFP package. Below is a summary of key pins and their functions:

For a complete pinout, refer to the CM108B datasheet provided by C-Media.

1. Power Supply: Connect the CM108B to a 5V power source. The internal LDO regulator will step down the voltage to 3.3V for internal operation.
2. USB Connection: Connect the USB_DP and USB_DM pins to the USB data lines of your host device (e.g., a computer or microcontroller).
3. Audio Input/Output:
   • Connect a microphone to the MIC_IN pin for audio input.
   • Connect speakers or headphones to the SPK_OUT_L and SPK_OUT_R pins for audio output.
4. Reset: Use the RESET_N pin to initialize the chip. Pull this pin low momentarily to reset the device.
5. GPIO Pins: Configure GPIO1 and GPIO2 for additional functionality, such as LED indicators or custom controls.

• Decoupling Capacitors: Place decoupling capacitors close to the power supply pins to reduce noise and ensure stable operation.
• USB Compliance: Ensure proper USB signal integrity by using appropriate trace routing and impedance matching for USB_DP and USB_DM lines.
• Audio Quality: Use high-quality components for audio input/output paths to minimize distortion and noise.
• Firmware: The CM108B may require firmware or driver installation on the host device for proper operation.

While the CM108B is primarily a USB audio controller, it can be interfaced with an Arduino UNO for GPIO control or other custom applications. Below is an example code snippet for toggling GPIO1:

// Example code for controlling GPIO1 on the CM108B using Arduino UNO

const int gpio1Pin = 7; // Connect GPIO1 of CM108B to Arduino pin 7

void setup() {
  pinMode(gpio1Pin, OUTPUT); // Set GPIO1 as an output
}

void loop() {
  digitalWrite(gpio1Pin, HIGH); // Turn GPIO1 ON
  delay(1000);                  // Wait for 1 second
  digitalWrite(gpio1Pin, LOW);  // Turn GPIO1 OFF
  delay(1000);                  // Wait for 1 second
}

Note: Ensure proper level shifting if the GPIO pins of the CM108B operate at 3.3V while the Arduino operates at 5V.

1. No Audio Output:

   • Verify that the SPK_OUT_L and SPK_OUT_R pins are properly connected to the speakers or headphones.
   • Check the USB connection and ensure the host device recognizes the CM108B.
   • Confirm that the correct audio drivers are installed on the host device.

2. Microphone Not Working:

   • Ensure the microphone is connected to the MIC_IN pin.
   • Check the microphone's compatibility and ensure it is powered correctly.
   • Verify the host device's audio input settings.

3. Device Not Recognized by USB Host:

   • Check the USB_DP and USB_DM connections for proper routing and soldering.
   • Ensure the USB cable is functional and compliant with USB 2.0 standards.
   • Reset the CM108B using the RESET_N pin.

4. Distorted Audio:

   • Use high-quality capacitors and resistors in the audio signal path.
   • Verify that the power supply is stable and free from noise.
   • Check the sampling rate and ensure it matches the host device's settings.

Q: Does the CM108B support stereo microphone input?
A: No, the CM108B supports mono microphone input only.

Q: Can the CM108B be used without additional firmware?
A: Yes, the CM108B is designed to work as a plug-and-play USB audio device on most operating systems. However, custom firmware may be required for advanced features.

Q: What is the maximum sampling rate supported by the CM108B?
A: The CM108B supports a maximum sampling rate of 48 kHz.

Q: Is the CM108B compatible with Linux?
A: Yes, the CM108B is compatible with Linux, as well as Windows and macOS, without requiring additional drivers in most cases.

Q: Can the GPIO pins be used for custom applications?
A: Yes, the GPIO pins can be configured for various purposes, such as controlling LEDs or interfacing with other peripherals.